1. Field of the Invention
The present invention relates to antenna devices preferably for use in RFID systems and short-range wireless communication systems, such systems performing communication through electromagnetic signals with appliances that are communication partners, and also relates to methods of setting the resonant frequency of such antenna devices.
2. Description of the Related Art
In RFID systems and short-range wireless communication systems, which have become widely used in recent years, in order to facilitate communication between mobile electronic appliances such as mobile telephones or between a mobile electronic appliance and a reader/writer, a communication antenna is mounted in each of the appliances. Among such antennas, an antenna to be mounted in a mobile electronic appliance is disclosed in Japanese Unexamined Patent Application Publication No. 2006-270681.
FIG. 1 is a view illustrating communication between a mobile information terminal 21 and a reader/writer as disclosed in Japanese Unexamined Patent Application Publication No. 2006-270681 when the mobile information terminal 21 and the reader/writer are in the vicinity of each other. In the example illustrated in FIG. 1, a magnetic field H, which is part of the electromagnetic waves radiated from a transmitting/receiving antenna unit 26 of the reader/writer, is affected by metal objects such as a battery pack 25 within a main body 22 of the terminal and is weakened due to reflection, absorption and the like. A metal layer 30 is arranged closer to a side onto which the electromagnetic waves are incident than a location of a communication surface CS of an antenna module 10. An induced current (eddy current) is generated due to the external magnetic field acting on the front surface of the metal layer 30 and a magnetic field H1 generated as a result of this current causes generation of an induced electric current in an antenna coil 15 of the antenna module 10.
In this example, the metal layer 30 is arranged close to and facing the antenna module 10 so as to cover part of the antenna coil 15 and thereby inductive coupling occurs between the transmitting/receiving antenna unit 26 of the reader/writer and the antenna coil 15 of the antenna module 10 via the magnetic field component H1 that arises around the metal layer 30.
The antenna device illustrated in FIG. 1 attempts to solve a problem in that when the antenna of the antenna device and the antenna on the communication partner side are brought close to each other and the distance therebetween becomes very small, the communication characteristics fluctuate greatly depending on the magnitude of positional displacement between the centers of the two antenna. In order to reduce the degree to which magnetic flux, which is attempting to link the antenna coil 15 of the antenna module 10 on the mobile information terminal 21 side and transmitting/receiving antenna unit 26 on the reader/writer side, is blocked by metal objects such as the battery pack 25 within the casing, the metal layer 30 is provided to induce magnetic flux in that region. Therefore, a substantial effect cannot be regularly or reliably obtained by changing the positions of shielding members such as the battery pack 25.
Furthermore, the metal layer 30 effectively does not regularly or reliably increase the communication range in a state in which the antenna device and the antenna on the communication partner side are spaced apart from each other.